Future mmW-based radio access technology, such as for example between a base station/access node (eNB) and a UE (user equipment) such as a user terminal, or between two UE:s, will heavily rely on beam-forming. This is primarily due to a desire to acquire an acceptable path loss due to the small aperture of single antennas at those high frequencies, but is also due to a desire to compensate for the progressively reduced power capability of power amplifier and increased noise figure of receivers as the frequency of operation is increased.
Radio links, e.g. point-to-point, wireless backhaul for eNB etc., is another application that exploits beam-forming, but is different in that they typically are considered as being fixed and not moving, as is the case for a UE communicating with an eNB.
Beam-forming exhibits spatial selectivity that can be beneficial in a multi-user scenario. But it also leads to requirements on accurate beam tracking, which means estimating direction of a received beam and steer the antenna accordingly, for the transmission link not to become a victim of that same selectivity. This can be a severe problem even when UE:s move slowly, in case the beams are very narrow, having a beam width of about just a few degrees.
Generally, beam tracking is required foremost not to lose a radio link and better still to maintain the quality of the radio link between any two nodes when there is a movement of at least one of the nodes. While a moving UE connected to an eNB appears to be the most obvious case also radio links with very narrow beams can benefit from beam tracking as tiny movements due to vibrations or wind may have a large impact on the link quality. Beam tracking can be based on a combination of techniques including RSSI measurements in different beam directions and motion detectors in a UE (or any node) that in turn are used to steer the antenna beam of that same device.
There is thus a problem related to that the movement of UE:s may be too fast to correct for in the UE only by means beam tracking based on measurements of received signal strength.
In any case, additional techniques that can improve beam tracking are desirable to allow for more narrow beams.
It therefore exists a need to provide a more accurate measurement of the direction of a received beam, and more specifically the deviation from the desired beam direction.